|TELNETD(8)||System Manager's Manual||TELNETD(8)|
telnetd — DARPA
telnet protocol server
telnetd program is a server which
supports the DARPA telnet interactive communication protocol.
Telnetd is normally invoked by the internet server
(see inetd(8)) for requests to connect to the telnet port
as indicated by the /etc/services file (see
may be used to start up
telnetd manually, instead of
through inetd(8). If started up this way,
port may be specified to run
telnetd on an alternate TCP port number.
telnetd program accepts the following
- This option may be used for specifying what mode should be used for
authentication. Note that this option is only useful if
telnetdhas been compiled with support for authentication, which is not available in the current version. The following values of authmode are understood:
- Turns on authentication debugging code.
- Only allow connections when the remote user can provide valid authentication information to identify the remote user, and is allowed access to the specified account without providing a password.
- Only allow connections when the remote user can provide valid authentication information to identify the remote user. The login(1) command will provide any additional user verification needed if the remote user is not allowed automatic access to the specified account.
- Only allow connections that supply some authentication information.
This option is currently not supported by any of the existing
authentication mechanisms, and is thus the same as specifying
- This is the default state. Authentication information is not required. If no or insufficient authentication information is provided, then the login(1) program will provide the necessary user verification.
- This disables the authentication code. All user verification will happen through the login(1) program.
- This option may be used for debugging purposes. It allows
telnetdto print out debugging information to the connection, allowing the user to see what
telnetdis doing. Repeated use of the option arranges composite debug reports. There are several possible values for debugmode:
- Prints information about the negotiation of telnet options.
- Prints the
optionsinformation, plus some additional information about what processing is going on.
- Displays the data stream received by
- Displays data written to the pty.
- Has not been implemented yet.
telnetdhas been compiled with support for encryption, then the
-edebugoption may be used to enable encryption debugging code.
- Disables the printing of host-specific information before login has been completed.
- This option may be used to specify a different login program. By default, /usr/lib/telnetlogin is used.
telnetdenables the TCP keep-alive mechanism to probe connections that have been idle for some period of time to determine if the client is still there, so that idle connections from machines that have crashed or can no longer be reached may be cleaned up.
- Disable reverse DNS lookups and use the numeric IP address in logs and REMOTEHOST environment variable.
- This option is only enabled if
telnetdis compiled with support for SecurID cards. It causes the
-soption to be passed on to login(1), and thus is only useful if login(1) supports the
-sflag to indicate that only SecurID validated logins are allowed. This is usually useful for controlling remote logins from outside of a firewall.
- Sets the IP type-of-service (TOS) option for the telnet connection to the value tos.
- This option is only valid if
telnetdhas been built with support for the authentication option. It disables the use of authtype authentication, and can be used to temporarily disable a specific authentication type without having to recompile
If the file /etc/issue.net is present,
telnetd will display its contents before the login
prompt of a telnet session (see issue.net(5)).
Telnetd operates by allocating a
pseudo-terminal device (see pty(4)) for a client, then
creating a login process which has the slave side of the pseudo-terminal as
the master side of the pseudo-terminal, implementing the telnet protocol and
passing characters between the remote client and the login process.
When a telnet session is started up,
telnetd sends telnet options to the client side
indicating a willingness to do the following telnet options, which are
described in more detail below:
DO AUTHENTICATION WILL ENCRYPT DO TERMINAL TYPE DO TSPEED DO XDISPLOC DO NEW-ENVIRON WILL SUPPRESS GO AHEAD DO ECHO DO LINEMODE DO NAWS WILL STATUS DO LFLOW DO TIMING-MARK
The pseudo-terminal allocated to the client is configured to
operate in “cooked” mode, and with
CRMOD enabled (see
Telnetd has support for enabling locally
the following telnet options:
- WILL ECHO
- When the
LINEMODEoption is enabled, a
WONT ECHOwill be sent to the client to indicate the current state of terminal echoing. When terminal echo is not desired, a
WILL ECHOis sent to indicate that telnetd will take care of echoing any data that needs to be echoed to the terminal, and then nothing is echoed. When terminal echo is desired, a
WONT ECHOis sent to indicate that telnetd will not be doing any terminal echoing, so the client should do any terminal echoing that is needed.
- WILL BINARY
- Indicates that the client is willing to send a 8 bits of data, rather than the normal 7 bits of the Network Virtual Terminal.
- WILL SGA
- Indicates that it will not be sending
IAC GA,go ahead, commands.
- WILL STATUS
- Indicates a willingness to send the client, upon request, of the current status of all TELNET options.
- WILL TIMING-MARK
- Whenever a
DO TIMING-MARKcommand is received, it is always responded to with a
- WILL LOGOUT
- When a
DO LOGOUTis received, a
WILL LOGOUTis sent in response, and the TELNET session is shut down.
- WILL ENCRYPT
- Only sent if
telnetdis compiled with support for data encryption, and indicates a willingness to decrypt the data stream.
Telnetd has support for enabling remotely
the following TELNET options:
- DO BINARY
- Sent to indicate that telnetd is willing to receive an 8 bit data stream.
- DO LFLOW
- Requests that the client handle flow control characters remotely.
- DO ECHO
- This is not really supported, but is sent to identify a 4.2BSD
telnet(1) client, which will improperly respond with
WILL ECHO.If a
WILL ECHOis received, a
DONT ECHOwill be sent in response.
- DO TERMINAL-TYPE
- Indicates a desire to be able to request the name of the type of terminal that is attached to the client side of the connection.
- DO SGA
- Indicates that it does not need to receive
IAC GA,the go ahead command.
- DO NAWS
- Requests that the client inform the server when the window (display) size changes.
- DO TERMINAL-SPEED
- Indicates a desire to be able to request information about the speed of the serial line to which the client is attached.
- DO XDISPLOC
- Indicates a desire to be able to request the name of the X windows display that is associated with the telnet client.
- DO NEW-ENVIRON
- Indicates a desire to be able to request environment variable information, as described in RFC 1572.
- DO LINEMODE
- Only sent if
telnetdis compiled with support for linemode, and requests that the client do line by line processing.
- DO TIMING-MARK
- Only sent if
telnetdis compiled with support for both linemode and kludge linemode, and the client responded with
WONT LINEMODE.If the client responds with
WILL TM,the it is assumed that the client supports kludge linemode. Note that the [
-k] option can be used to disable this.
- DO AUTHENTICATION
- Only sent if
telnetdis compiled with support for authentication, and indicates a willingness to receive authentication information for automatic login.
- DO ENCRYPT
- Only sent if
telnetdis compiled with support for data encryption, and indicates a willingness to decrypt the data stream. issue.net(5)).
- TELNET PROTOCOL SPECIFICATION
- TELNET OPTION SPECIFICATIONS
- TELNET BINARY TRANSMISSION
- TELNET ECHO OPTION
- TELNET SUPPRESS GO AHEAD OPTION
- TELNET STATUS OPTION
- TELNET TIMING MARK OPTION
- TELNET EXTENDED OPTIONS - LIST OPTION
- TELNET END OF RECORD OPTION
- Telnet Window Size Option
- Telnet Terminal Speed Option
- Telnet Terminal-Type Option
- Telnet X Display Location Option
- Requirements for Internet Hosts -- Application and Support
- Telnet Linemode Option
- Telnet Remote Flow Control Option
- Telnet Authentication Option
- Telnet Authentication: Kerberos Version 4
- Telnet Authentication: SPX
- Telnet Environment Option Interoperability Issues
- Telnet Environment Option
Some TELNET commands are only partially implemented.
Binary mode has no common interpretation except between similar operating systems (Unix in this case).
The terminal type name received from the remote client is converted to lower case.
Telnetd never sends TELNET
IAC GA (go ahead) commands.
The source code is not comprehensible.
|December 29, 1996||Linux NetKit (0.17)|